The phosphorylation of proteins plays a key role in the transduction of extracellular signals into the cell. The enzymes, called kinases, which effect such phosphorylations are targets for the action of growth factors, hormones, and other agents involved in cellular metabolism, proliferation and differentiation. One of the major signal transduction pathways involves the enzyme protein kinase C (PKC), which is known to have a critical influence on cell proliferation and differentiation. PKC is activated by diacylglycerols (DAGs), which are metabolites released in signal transduction.
Interest in PKC was stimulated by the finding that PKC is the major, and perhaps only, cellular receptor through which a class of tumor-promoting agents called phorbol esters exert their pleiotropic effects on cells [Gescher et al., Anti-Cancer Drug Design 4:93-105 (1989)]. Phorbols capable of tumor production can mimic the effect of DAG in activating PKC, suggesting that these tumor promoters act through PKC and that activation of this enzyme is at least partially responsible for the resulting tumorigenesis [Parker et al., Science 233:853-866 (1986)].
Experimental evidence indicates that PKC plays a role in growth control in colon cancer. It is believed that specific bacteria in the intestinal tract convert lipids to DAG, thus activating PKC and altering cell proliferation. This may explain the correlation between high dietary fat and colon cancer [Weinstein, Cancer Res. (Suppl.) 51:5080s-5085s (1991)]. It has also been demonstrated that a greater proportion of the PKC in the colonic mucosa of patients with colorectal cancer is in an activated state compared to that of patients without cancer [Sakanoue et al., Int. J. Cancer 48:803-806 (1991)].
Increased tumorigenicity is also correlated with overexpression of PKC in cultured cells inoculated into nude mice. A mutant form of PKC induces highly malignant tumor cells with increased metastatic potential.
Sphingosine and related inhibitors of PKC activity have been shown to inhibit tumor cell growth and radiation-induced transformation in vivo [Endo et al., Cancer Research 51:1613-1618 (1991); Borek et al., Proc. Natl. Acad. Sci. 88:1953-1957 (1991)]. A number of experimental or clinically useful anti-cancer drugs show modulatory effects on PKC. Therefore, inhibitors of PKC may be important cancer-preventive or therapeutic agents. PKC has been suggested as a plausible target for more rational design of conventional anti-cancer drugs [Gescher, A. and Dale, I.L., Anti-Cancer Drug Design, 4:93-105 (1989)].
Experiments also indicate that PKC plays an important role in the pathophysiology of hyperproliferative skin disorders such as psoriasis and skin cancer. Psoriasis is characterized by inflammation, hyperproliferation of the epidermis and decreased differentiation of cells. Various studies indicate a role for PKC in causing these symptoms. PKC stimulation in cultured keratinocytes can be shown to cause hyperproliferation. Inflammation can be induced by phorbol esters and is regulated by PKC. DAG is implicated in the involvement of PKC in dermatological diseases, and is formed to an increased extent in psoriatic lesions.
Inhibitors of PKC have been shown to have both antiproliferative and antiinflammatory effects in vitro. Some antipsoriasis drugs, such as cyclosporine A and anthralin, have been shown to inhibit PKC. Inhibition of PKC has been suggested as a therapeutic approach to the treatment of psoriasis [Hegemann, L. and G. Mahrle, Pharmacology of the Skin, H. Mukhtar, ed., p. 357-368, CRC Press, Boca Raton, Fla., 1992].
PKC is not a single enzyme, but a family of enzymes. At the present time at least seven isoforms (isozymes) of PKC have been identified: .alpha., .beta., .gamma., .delta., .epsilon., .zeta. and .eta.. These isozymes have distinct patterns of tissue and organ localization (see Nishizuka, Nature, 334:661-665 (1988) for review) and may serve different physiological functions. For example, PKC-.gamma. seems to be expressed only in the central nervous system. PKC-.alpha. and -.beta. are expressed in most tissues, but have different patterns of expression in different cell types. For example, both PKC-.alpha. and PKC-.beta. are expressed in, and have been purified from, human epidermis. While PKC-.alpha. has been detected mainly in keratinocytes of the basal layers of the epidermis, PKC-.beta. is found mainly in the middle layers of the epidermis and Langerhans cells. PKC-.eta. has been found predominantly in the skin and lungs, with levels of expression much higher in these tissues than in the brain. This is in contrast to other members of the PKC family which tend to be most abundantly expressed in the brain [Osada et al., J. Biol. Chem. 265:22434-22440 (1990)]. Another PKC isozyme, PKC-.eta., is believed to play a critical role in control of proliferative cascades. This was demonstrated by using antisense RNA, peptide inhibitors or a 15-mer phosphorothioate antisense oligonucleotide targeted to the AUG of Xenopus PKC-.eta. to deplete PKC-.eta. levels in Xenopus oocytes. These depleted oocytes were shown to be resistant to maturation in response to insulin, while the maturation pathway activated by progesterone was not affected. WO 93/20101. While the PKC isozymes listed here are preferred for targeting by the present invention, other isozymes of PKC are also comprehended by the present invention.
It is presently believed that different PKC isozymes may be involved in various disease processes depending on the organ or tissue in which they are expressed. For example, in psoriatic lesions there is an alteration in the ratio between PKC-.alpha. and PKC-.beta., with preferential loss of PKC-.beta. compared to normal skin [Hegemann, L. and G. Mahrle, Pharmacology of the Skin, H. Mukhtar, ed., p. 357-368, CRC Press, Boca Raton, Fla., 1992].
Even for a given isozyme, there may be multiple RNA transcripts expressed from a single gene. In the case of PKC.alpha., for example, two mRNA transcripts are seen: a long (approximately 8.5 kb) transcript and a short (approximately 4 kb) transcript. Multiple PKC.alpha. transcripts are produced from the murine and the bovine PKC.alpha. genes as well. The ratio between the long and short transcripts varies between species and is believed to vary between tissues as well. In addition, there may be some correlation between this ratio and the proliferative state of cells.
Although numerous compounds have been identified as PKC inhibitors (see Hidaka and Hagiwara, Trends in Pharm. Sci. 8:162-164 (1987) for review), few have been found which inhibit PKC specifically. While the quinoline sulfonamide derivatives such as 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) inhibit PKC at micromolar concentrations, they exhibit similar enzyme inhibition kinetics for PKC and the CAMP-dependent and cGMP-dependent protein kinases. Staurosporine, an alkaloid product of Streptomyces sp., and its analogs, are the most potent in vitro inhibitors of PKC identified to date. However, they exhibit only limited selectivity among different protein kinases [Gescher, Anti-Cancer Drug Design 4:93-105 (1989)]. Certain ceramides and sphingosine derivatives have been shown to have PKC inhibitory activity and to have promise for therapeutic uses, however, there remains a long-felt need for specific inhibitors of the enzymes.
There is also a desire to inhibit specific PKC isozymes, both as a research tool and as treatment for diseases which may be associated with particular isozymes. Godson et al. [J. Biol. Chem. 268:11946-11950 (1993)] recently disclosed use of stable transfection of antisense PKC-.alpha. cDNA in cytomegalovirus promotor-based expression vectors to specifically decrease expression of PKC-.alpha. protein by approximately 70%. It was demonstrated that this inhibition causes a loss of phospholipase A.sub.2 -mediated arachidonic acid release in response to the phorbol ester PMA. Attempts by the same researchers at inhibiting PKC activity with oligodeoxynucleotides were ultimately unsuccessful due to degradation of oligonucleotides.